It is a standard practice, in operating oil-lubricated vacuum-pumps, to introduce a predetermined quantity of oil or other lubricant into the pump chamber at the beginning of each compression cycle. The oil performs several functions. First, the oil serves to lubricate the component parts of the vacuum-pump that come into contact with one another. Second, the oil provides a corrosion-resistant coating on interior surfaces of the pump. Third, the oil stream acts as a medium for ridding the pump both of chemical and particulate impurities, and of heat, i.e. of cleansing the pump chamber and cooling the pump. Finally, the oil serves to improve the seal between the intake and discharge regions of the pump.
Each of these functions may require a different quantity of oil. Thus, the determination of the quantity of oil to be introduced in the pump chamber before each compression event is necessarily based on a compromise between the quantities required for the respective oil functions.
Furthermore, it is known that the oil requirements of a vacuum-pump vary under different operating conditions. For example, larger quantities of oil are required during the initial operating stages of the pump, in which output is high, than are required in subsequent operating stages, which have lower outputs. Optimally, the oil should perform each of its designated functions under a wide range of operating conditions.
DE-AS 11 79 666 discloses a single-stage rotary-piston vacuum pump provided with a control mechanism that actuates a supplemental oil feed. The control mechanism is responsive to pressure in the compression space of the pump chamber. The pressure in the pump chamber is above atmospheric pressure during high output pump stages (e.g. during pump start-up), and below atmospheric pressure during low output stages (also referred to as the "ultimate pressure mode"). The supplemental oil feed is controlled by means of a piston that is actuated by pressure variation in the pump chamber, and admits greater quantities of oil into the pump chamber during high pressure operational stages.
This type of supplemental oil feed is unsuitable for use with a high-vacuum stage of a multi-stage vacuum pump, due to its lack of operational precision, and furthermore since its operation is dependent solely upon output pressure.
In modern vacuum-pumps, discharge pressure alone is an inadequate indicator for triggering lubrication. For example, elevated discharge pressure may occur when downstream oil filters are overloaded with filtrate. Known supplemental oilers have no way of recognizing such a condition. If this condition were to occur during the ultimate pressure mode, increased quantities of oil would flow into the pump chamber during pump operational stages that actually require smaller quantities of oil. Such over-supply of oil is undesirable, since it may interfere with pump operation.
It is therefore apparent that a need exists for a method and apparatus for delivering oil into a pump chamber of a high vacuum stage of a multi-stage pump which provides a precise, operationally dependent delivery of oil.